The invention relates to a device for examining friction conditions on friction partners, by which a normal force is impressed between two friction partners to be tested, and determined by means of a normal force sensor, and by which, furthermore, the tangential force acting during a relative movement between the friction partners is determined by means of a friction force sensor.
The device serves for determining low friction values with high accuracy.
Different designs of devices for exactly testing friction conditions are known in the prior art.
A device designed in the form of a pin-and-disk tribometer is described in MITTMANN, CZICHOS: Friction Measurements and Surface Examinations on Plastic/Metal Sliding Partners; Materialprxc3xcfung 17, 1975, No. 10, October, pages 366-367. The tests are carried out in that connection in a closed test chamber that can be tempered and flushed with defined gases. The disk is driven at constant speed by a tachometer-controlled dc motor via an interconnected gearing. A cylindrical pin made of Polytetrafluorothylene (PTFE), High Density Polyethylene (HDPE) or Polyethylene Terephthalate (PETP) is cut prior to the test with a microtome. The pin is aligned parallel with the surface of the disk with a pair of observation binoculars. An arrangement comprising four flexing spars is employed as the element absorbing the force.
The drawback in this connection is that the device requires much expenditure and that it can be used only within a limited range of forces.
A device for testing friction conditions, in which tangential and normal forces are transmitted with an elastic flexural element, is known from BURGER J F ET AL, Proc. 9th Annual Workshop on Micro mechanical Systems, San Diego 1996 (IEEE), pp 99 to 104.
A force sensor for instruments used in surface analysis is known from U.S. Pat. No. 5,661,235, where biaxial spring elements are employed for transmitting tangential and normal forces.
The known arrangements have the drawback that they either require a costly structure or permit only low measuring accuracy.
Therefore, the invention is based on the problem of proposing a device of the type specified above that permits measurements with high accuracy and good reproducibility with simple means.
In a preferred embodiment of the invention, the device has an elastic flexing element made of glass, which contains a plurality of spring arrangements with different spring stiffness properties. Integrated in the elastic element is a soft parallel spring element, which is provided with stops that are integrated in the elastic element for impressing the tangential movement, and a harder leaf spring element for impressing the normal force, the leaf spring element acting perpendicular in relation to the two parallel spring elements.
The points of application of force and the sensors are located in each case)on opposite parts of the elastic flexing element.
Using a frontal drive, a motion is generated in the direction of the area normal to the friction surfaces that are in contact with one another. An adjusting device is usefully integrated in the frontal drive for aligning the two body planes in parallel. The deflection of the relatively hard (horizontal) part of the elastic flexing element, such deflection being generated by the movement of the frontal drive, has the effect that the two friction surfaces will be disposed one on top of the other with an adjustable, exactly measurable, and low normal force (ranging from a few xcexcN to about 10 N). The normal force is converted into an electrical signal with a force sensor, preferably by means of an optical distance sensor. For the determination of the friction conditions it is necessary to determine the force acting tangentially. A relative movement is impressed for said purpose with the tangential drive. The soft elastic tangential guide and the application of force connected therewith ensue not directly in dependence of the friction conditions from the driving force, but rather from a difference in the distance obtained between the part of the elastic tangential guide connected to the point of application of the forcer and the part of the tangential guidance connected to the tangential drive. Said difference in distance is converted into an electrical signal with the help of a sensor, preferably by means of an optical distance sensor, and supplied to a signal processing unit together with the signal from the normal force sensor.